Purification of dimethylterephthalate



Feb- 24, 1959 J'. T. coRcoRAN ETAL 2,875,237

PURIFICATION OF DIMETHYLTEREPHTHALATE Filed June 7, 1957 Crude DMT TTOR/VE United States Patent O 2,875,237 PURLFICATION or DnmTHYLTEREPHrHALATE James T. Corcoran, Brownsville, Tex.,`andV Kenneth C.

Peterson, Chicago, Ill., assignors to Standard Oil lCompany, Chicago, Ill., a corporation of Indiana Application June 7, 1957, seria1N0.664,39o 10 4 Claims. (cl. 26o-47s) This invention relates to the `preparation of extremely high purity dimethylte ephthalate from crude dimethyl-` terephthalate.

The synthetic fiber industry utilizes dimethylterephthalate as an intermediate; for this purpose the dimethylterephthalate mustbe of extremely high purity as measured by the acid number and h melting point of the material. It is desired that the dimethylterephthalate have an acid number as close to zero as possible and not in excess of about 0.05; the melting point desired is about 140.629 C. with a tolerance of about i0.02 C. 25

Dimethylterephthalate is made by the reaction of methanol and terephthalic acid withor without `the presence of a metallic cataylst. The reaction cannotbe forced to complete conversion of the terephthalic acid. The crude dimethylterephthalate contains normally among other materials unreacted acid,jmono-methylterephthalate, impurities which were present in the acid charged, and catalyst if this was used. lt has been found that extremely high purity dimethylterephthalate can be produced by distillation of the crude ester but at relatively low yields. L

An object of the invention is a process for preparing extremely high purity dimethylterephthalate in high yield. Another object of the invention is a process for purifying the crude dimethylterephthalate to produce dimethylterephthalate of extremely high purity in a simple procedure involving only physical separations. Other objects will become apparent in the course of the detailed description. p

Figure l shows in block diagram one form of the instant invention wherein distillation is the final step in the purification process. p

Figure 2 shows another embodiment of the invention wherein `a distillation operation `forms the iirst step in` the purification process. i 50 In Figure 1 there is set out in block form one embodiment of the process. In this embodiment "the crude DMT from source 11 is passed by way of line `12 into vessel 13. The crude dimethylterephthalate which forms the charge to the instant process is characterized by an acid number (mg. KOH per g. of material) between 0.1 and about 50.

It is usual for the crude dimethylterephthalate from the reaction which have proceeded to about equilibrium to have an acid number in the region of 2 to 20. The crude dimethylterephthalate (DMT) charge may be either that taken directly from the esteritication operation or may be material which has been partially purified either by distillation or by a recrystallization from methanol procedure.

Liquid methanol from source 16 is passed by way of 65 line 17 into vessel 13. The liquid methanol is preferably essentially anhydrous but may contain a small amount of water. `Ihe amount of liquid methanol charged to vessel 13 is suiiicient to dissolve enough of the impurities 70 present so that the DMT recovered from vessel 13 has an acid number lower than that of the crude DMT from syof the distillate increases above about 0.05.

ice

t Patented Feb. 24, 1959 source 11. 'Ihe amount of methanol charged, however, is not enough to dissolve any substantial amount of DMT. In general, between about 1 and 10 parts by weight of liquid methanol are charged to vessel 13 per part of crude DMT charged to vessel 13. Vessel 13 is provided with agitation means Asuch as a propeller, turbine or inert gas.

When using inert gas `agitation means are provided for recovering methanol stripped from vessel by the inert gas. The DMT and methanol form a slurry in vessel 13; the contacting ofthe liquid methanol and crude DMT is continued for atime` such that the acid number of the contacted DMT is substantially `reduced below that of the crude DMT. When the acid number of the crude DMT is in the region of 2-20 it is preferable to use between about2 and 6 parts by weight of liquid methanol per part of the crude DMT.Y

The upper temperature of contacting in vessel 13 is below the boiling point of methanol at the particular pressure existing in vessel 13. Quite low temperatures may be used. In general, the slurry operation in vessel 13 is carried out at `a temperature between about 10 C. and 60 C.; the higher temperatures usually will be used.

The time of contacting of the liquid methanol and crude DMT in vessel 13 partially determines the reduction in acid number obtained therein. While the temperatureof contacting does somewhat decrease the minimum time needed in general, the amount of methanol used determines the minimum amount of contacting. It is desirable touse a time of atleast about 5 minutes; contacting times of an hour or more may be used if desired. i 1

The slurry from vessel 13 is passedbyway of line 19 into lter zone 21. Filter zone 21 is adapted for removal of the liquid'methanol solution from the contacted solid DMT. An ordinary suction lter ora centrifuge may be used. The liquid methanol solution comprising methanol chargedfand `dissolved impurities from thel crude DMT is withdrawn from lilter zone 21 by way of line 22. The liltered solid DMT is passed by` way of line 23 into distillation zone 24. Distillation zone Z4 t may be a batch operation or a continuous operation. In

distillation zone 24 the filtered DMT is distilled to produce overhead a fraction `which comprises DMT and impurities which is shownas being withdrawn by way of line 26; The material withdrawn by` way of line 26 has an acid number greater than 0.05; in operation of a batch distillation-A the overhead product will be cut when the overhead reaches an` acid number on the order of 0.05. The product purified DMT is then distilled. until vapors either Stop evolving or, in some cases, the acid number Purified DMT is shown as being withdrawn by way of line 27. A smallamount of very high boiling material is withdrawn from zone 24 by way of line 28.

p It is `to be understood thatthe amount of methanol charged to vessel 13 `will be determined in. part by the acid `number of thel crude DMT charged. `When it is desired to have a `maximum recovery of puriiiedfDMT from distillation zone 24 the lower the acid number of the filtered DMT the more the amount of material produced as DMT.

Some of the material produced from the distillation zone and discarded through line 26 may be recycled to vessel 13 in order to recover some of the DMT content. Not all of this material may be recycled since there will be a build-up of impurities in the system.

Example I Crude dimethylterephthalate produced by catalytic esterication which had an acid number of 20 was charged to a vessel and slurried with 3 pounds of anhydrous methanol per pound of crude DMT. The slurrying operation was carried out at about 25 C. for a total time The contacted of 30 minutes-using a propeller stirrer. DMT was recovered by suction filtration of the slurry and was freed of entrained methanol in a Vacuum dryer. The `yield of filtered DMT was 91% of the crudeDMT and had an acidnumb'er 'of 13.l The filtered DMT was charged to-a batch distillation column having about 20 actual platesfVThe bottoms .product represented about 3% ofthe charge'to the column. Eighty-nine percent of the Vmaterial taken overhead had an acid number of less than0.05; the melting point of this DMT was 140.64. This puriii'e'd DMT meets the requirements for synthetic riber grade dimethylterephthalate.

In Figure 2 Vthere is shown another embodiment of the purification process. vIn this embodiment crude dimethylterephthalate fromsource 51 is passed by way of line 52 into distillation zone 53, which Zone is identical to zone 24.` In this embodiment the crude DMT has a maximum acid number of about 25. In distillation Vzone 53 an overhead fraction containing most of the'impurities is Withdrawn by way of line 54. A bottoms product of material boiling above DMT is Withdrawn by Way of line 56. A DMT overhead fraction of extremely high purity is shownas being withdrawn by way of line 57.

` The fraction containing the concentration of impure DMT is passed by way of line 54 into vessel 59`which is identical with vessel 13. Liquid methanol from source 61 is passed by waylof line 62 into vessel 59. Here between about 2 and 10 parts by weight of liquid methanol are charged per'part of said distilled'crude DMT charged into vessel 5,9.l The temperature in vessel 59 is maintained below the boiling point of methanol at the pressure utilized in` vessel 59. Usually thepressure will be atmospheric and the temperature between about C. and 60 C.; since purified DMT is to be produced without distillation lvesselSS is operated at temperatures in the regionv of the boilingl point of methanol for'example, 5060 C. Y

The slurry is Aagitated in vessel 59 for a time such that the solid DMT recovered from the slurry has an acid number Iof not more than0.05. InY general, contacting times will be on the order of 10-60 minutes.

The slurry from Vvessel 59 is passed by way of line 64 into filter 'zone 66 which is identical with filter zone 271. The liquid methanol solution is withdrawn from zone 66 by way of line 67.' Puried DMT which is vof extremely high purity is withdrawn from zone -66 by way of line 68. The material from lines 57 and 68` may be blended to produce the final product. Particularl/When the amount of distilled impure DMT is small,`the blend ing operation lis most conveniently performed indistillation zone 53 by cycling the material from line 68 thereto.

- By this procedure only one stream of pure DMTis produced and this from line 57.

Example 2 In this'exam-ple the crude DMT to be purified had Yan acid number of 8. The crude DMT was charged to la distillation column providing about plates. Three percent of the material charged tothe distillation was discarded as'bottoms. Sixty-live percent of the crude DMT was recovered from the distillation zone as extremely high purity material having an acid number less than 0.05 and a melting point of 140.61 C. The distillate fraction having an acid number higher than 0.05 was slurred with 3 pounds of anhydrous methanol per pound of the distilled crude DMT; the slurrying operation was carried out at 60 C. for a time of l5 minutes. The slurry was cooled to room temperature and the. solid DMT was recovered by suction filtration. The entrained liquid methanol solution was removed by vacuum drying. The yield of purified DMT from theslurrying operation was 97% of the distilled crude DMT. The purified DMT from the slurrying operation had an acid number of 0.02 and a melting point of 140.61 C.

Thus having described the invention, what is claimed is: l. A process for preparing extremely high purity dimethylterephthalate from crude dimethylterephthalate having an acid number between 0.1 and about 50 which process comprises contacting at a temperature between about 10 and 60 C. said crude DMT with betweenV about 1 and l0 parts by weight of liquid methanol per part of said crude DMT vwhereby said crude DMT forms a slurry in said liquid methanol, for a time of at least about 5 minutes such that the acid number of the contacted DMT is substantially reduced, iiltering solid DMT of lowered acid number from a liquid methanol solution, distilling said filtered vDMT to recover a `distilled DMT fraction characterized by an acid number of not more than about g 0.05 and a melting point of about 140.62 C.

. terizerd'4 by an acid number of not more than about 0.05

and a melting point of about 140.62 C., (2) contacting at a temperature between about 50 and 60 C. said distilled crudel DMT with between about 2 and 10 parts by weight of liquid methanol per part of said distilled crude DMT'whereby said distilled crude DMT forms a slurry in said liquid methanol for a time of at least about 10 minutes such lthat the solid DMT recovered from said slurry has an acid number of not more than about 0.05, (3) iiltering solid DMT from a liquid methanol solution, and (4) recovering adhering methanol from said solid DMT to'obtain al DMT product characterised by an acid number of not more than about 0.05 and a melting point of about 140.62" C. y

4. The process of claimy 3 wherein said methanol usage is between about 3 and 6 parts Vby weight per part of distilled crude DMT.

References Cited in the file of this patent VvUNITED STATES PATENTS 2,459,014" i Cavanaugh et al. Jan. 11, 1949 2,773,090 Leibu Dec. 4, 14956 FOREIGN PATENTS 1,119,564 France Apr. s, 1956 

1. A PROCESS FOR PREPARING EXTREMELY HIGH PURITY DIMETHYLTEREPHTHALATE FROM CRUDE DIMETHYLTEREPHTHALATE HAVING AN ACID NUMBER BETWEEN 0.1 AND ABOUT 50 WHICH PROCESS COMPRISES CONTACTING AT A TEMPERATURE BETWEEN ABOUT 10* AND 60* C.SAID DMT WITH BETWEEN ABOUT 1 AND 10 PARTS BY WEIGHT OF LIQUID METHANOL PER PART OF SAID CRUDE DMT WHEREBY SAID CRUDE DMT FORMS A SLURRY IN SAID LIQUID METHANOL FOR A TIME OF AT LEAST ABOUT 5 MINUTES SUCH THAT THE ACID NUMBER OF THE CONTACTED DMT IS SUBSTANTIALLY REDUCED, FILTERING SOLID DMT OF LOWERED ACID NUMBER FROM A LIQUID METHANOL SOLUTION, DISTILLING SAID FILTERED DMT TO RECOVER A DISTILLED DMT FRACTION CHARACTERIZED BY AN ACID NUMBER OF NOT MORE THAN ABOUT 0.05 AND A MELTING POINT OF ABOUT 140.62* C.
 3. A PROCESS FOR PREPARING EXTREMELY HIGH PURITY DIMETHYLTEREPHTHALATE FROM CRUDE DIMETHYLTERPHTHALATE HAVING AN ACID NUMBER BETWEEN 0.1 AND ABOUT 25 WHICH PROCESS COMPRISES (1) DISTILLING SAID CRUDE DMT TO OBTAIN A DISTILLED CRUDE DMT FRACTION HAVING AN ACID NUMBER IN EXCESS OF 0.1 AND A DISTILLED DMT FRACTION CHARACTERIZED BY AN ACID NUMBER OF NOT MORE THAN ABOUT 0.025 AND A MELTING POINT POINT OF ABOUT 140.62* C,.(2) CONTACTING AT A TEMPERATURE BETWEEN ABOUT 50* AND 60* C., SAID DISTILLED CRUDE DMT WITH BETWEEN ABOUT 2 AND 10 PARTS BY WEIGHT OF LIQUID METHANOL PER PART OF SAID DISTILLED CRUDE DMT WHEREBY SAID DISTILLING CRUDE DMT FORMS A SLURRY IN SAID LIQUID METHANOL FOR A TIME OF AT LEAST ABOUT 10 MINUTES SUCH THAT THE SOILD DMT RECOVERED FROM SAID SLURRY HAS AN ACID DMT FROM A LIQUID METHANOL SOLUTION, (3) FILTERING SOILD DMT FROM A LIQUID METHANOL SOLUTION, AND (4) RECOVERING ADHERING METHANOL FROM SAID SOLID DMT TO OBTAIN A DMT PRODUCT CHARACTERISED BY AN ACID NUMBER OF NOT MORE THAN ABOUT 0.05 AND A MELTING POINT OF ABOUT 140.62* C. 